Oversampling sigma-delta (ΣΔ) modulator is suitable for high resolution analog-to-digital convertor (ADC) or digital-to-analog convertor (DAC). For example, the sigma-delta DAC comprises an upsampling circuit, a filter, a quantizer, a digital-to-analog convertor and a low pass filter (LPF).
In order to increase the resolution of the sigma-delta DAC, it can be achieved by increasing the oversampling rate, increasing the order of filter, or even increasing the number of bits within the quantizer. Higher oversampling rate has a drawback of consuming more power. Higher filter order would cause energy of the out-of-band noise to be larger, and increase the cost of the backend analog low pass filter. In addition, even though more number of bits within the quantizer would reduce the out-of-band noise, however, under a condition of limited number of quantization bits, energy of the out-of-band noise is still high. Hence, the LPF with specific corner frequency is required to filter out the out-of-band noise.
The LPF is composed of an operational amplifier, resistor(s) and capacitor(s). Since noise is proportional to the resistance in the LPF, to achieve high signal-to-noise ratio (SNR), the resistance of the resistor is required to be small. However, to maintain the corner frequency of the LPF, the LPF needs the capacitor with large capacitance, which requires too large circuit area.
Therefore, it is necessary to improve the prior art.